1. Field of the Invention
The present invention relates to an ultrasonic transducer arranged to perform signal conversion between an ultrasonic signal and an electric signal.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2001-128292 discloses an ultrasonic transducer including a piezoelectric device that is disposed on an inner bottom surface of a tubular outer case and a directivity control member that is disposed inside the outer case.
In the ultrasonic transducer disclosed in Japanese Unexamined Patent Application Publication No. 2001-128292, the directivity control member arranged to control the shape of an ultrasonic beam is in close contact with the inner bottom surface of the outer case to which the piezoelectric device is attached, in order to flatten the ultrasonic beam depending on the intended use of the ultrasonic transducer, e.g., object detection and distance measurement.
The directivity control member is a member including a hole having long axis extending in one of the planar (two-dimensional) directions. By arranging the directivity control member in close contact with the inner bottom surface of the outer case, an effective vibration region of ultrasonic waves is increased in the long-axis direction of the hole of the directivity control member, and the effective vibration region of ultrasonic waves is decreased in the short-axis direction of the hole of the directivity control member, i.e., in a direction substantially perpendicular to the long-axis direction. Furthermore, as a contact area between the bottom surface of the outer case and a surface (hereinafter referred to as an “ultrasonic vibration acting surface”) of the directivity control member arranged to face the inner bottom surface of the outer case increases, a larger mass is applied to a contact portion of the outer case, which restrains vibration of the outer case. Hereinafter, such a mass is referred to as a “restraint mass”. Thus, by configuring the effective vibration region to have different sizes between the long-axis direction and the short-axis direction of the hole of the directivity control member such that the restraint mass applied to the bottom surface of the outer case is increased in portions of the outer case on both sides of the hole along the long axis, the bottom surface of the outer case, which defines a vibrating surface, is subjected to anisotropy between the long-axis direction and the short-axis direction of the hole of the directivity control member. Such a mechanism is effective to flatten the ultrasonic beam.
However, the above-described related art has the following problems. The restraint mass applied from the ultrasonic vibration acting surface of the directivity control member to the bottom surface of the outer case is not rotationally symmetrical with respect to any angle. This implies that the restraint mass contributes to flattening the beam shape, but simultaneously causes large vibrations in a bending mode, i.e., a vibration mode in which the effective vibration region is alternately distorted in the long-axis direction and the short-axis direction. In other words, undesired vibrations, i.e., higher-order spurious vibrations, are generated in addition to the basic vibration. Because the undesired vibrations have frequencies that are close to resonance frequencies of the basic vibration, the undesired vibrations also tend to be excited together with the basic vibration. Consequently, the vibrations in the undesired vibration mode continue to vibrate, which adversely affects a reverberation characteristic.
If the undesired vibration mode continues for an extended period of time, the piezoelectric device continues to generate electric signals with vibrations caused by the reverberation. Therefore, an electric signal generated with the vibration of the piezoelectric device, which is caused by ultrasonic waves reflecting from an obstacle, is obscured in the electric signals generated with the vibrations caused by the reverberation. Accordingly, the ultrasonic waves reflecting from the obstacle cannot be accurately detected.
The generation of the undesired vibrations can be effectively suppressed by coating a damping material, such as a silicone resin or a urethane resin, over the bottom surface of the outer case, which includes the piezoelectric device disposed thereon, other than the effective vibration region. However, in an ultrasonic transducer having such an arrangement, the damping material absorbs not only the undesired vibrations, but also the basic vibration because the damping material is coated near the effective vibration region of the piezoelectric device. This results in a reduction in the sensitivity of the ultrasonic transducer.